fix(materials): universal FailedMaterial sentinel for unmatched mesh objects

- export_gltf.py: replace single-material fallback (only fired when
  len(appended)==1) with a universal sentinel that appends
  SCHAEFFLER_059999_FailedMaterial unconditionally and assigns it to
  every mesh object not matched by name-based lookup.
  Also adds in-memory magenta fallback if library append fails.
  Removes 2 temporary [DEBUG] print lines from investigation.

- blender_render.py: add FailedMaterial assignment inside
  _apply_material_library() for unmatched parts (was log-only before).
  Includes copy-on-write guard (users > 1) matching existing pattern.

Also added alias 'Stahl; Durotect CMT' (semicolon) → Durotect-Blue
to cover STEP files using semicolon separator instead of comma.

Verified: 23/25 objects matched correctly, 2 ISO8734 dowel pins
(empty material) receive SCHAEFFLER_059999_FailedMaterial as sentinel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

render-worker/scripts/export_gltf.py

@@ -19,6 +19,8 @@ import json
 import sys
 import traceback
 
+FAILED_MATERIAL_NAME = "SCHAEFFLER_059999_FailedMaterial"
+
 
 def parse_args() -> argparse.Namespace:
     argv = sys.argv
@@ -268,26 +270,49 @@ def main() -> None:
                     assigned += 1
                     assigned_names.add(obj.name)
                 else:
-                    pass  # name-matching miss — may be covered by single-material fallback below
+                    pass  # unmatched → will receive FailedMaterial sentinel below
             print(f"Material substitution: {assigned}/{len(mesh_objects)} mesh objects assigned")
 
-            # Single-material fallback: if only one library material was loaded,
-            # apply it to every object that name-matching missed.
-            # (mat_map_lower may contain unresolvable pass-through values like
-            # "Stahl; Durotect CMT", so checking appended is more reliable.)
-            if len(appended) == 1:
-                default_mat_name, default_mat = next(iter(appended.items()))
-                if default_mat:
-                    fallback = 0
-                    for obj in mesh_objects:
-                        if obj.name not in assigned_names:
-                            if obj.data.users > 1:
-                                obj.data = obj.data.copy()
-                            obj.data.materials.clear()
-                            obj.data.materials.append(default_mat)
-                            fallback += 1
-                    if fallback:
-                        print(f"Single-material fallback: applied '{default_mat_name}' to {fallback} unmatched objects")
+            # Universal FailedMaterial sentinel: assign SCHAEFFLER_059999_FailedMaterial
+            # to every mesh object that was not matched by name-based lookup above.
+            # Replaces the old single-material fallback that only fired when len(appended)==1.
+            failed_mat = None
+            try:
+                bpy.ops.wm.append(
+                    filepath=f"{args.asset_library_blend}/Material/{FAILED_MATERIAL_NAME}",
+                    directory=f"{args.asset_library_blend}/Material/",
+                    filename=FAILED_MATERIAL_NAME,
+                    link=False,
+                )
+                if FAILED_MATERIAL_NAME in bpy.data.materials:
+                    failed_mat = bpy.data.materials[FAILED_MATERIAL_NAME]
+                    print(f"Appended sentinel material: {FAILED_MATERIAL_NAME}")
+                else:
+                    print(f"WARNING: sentinel '{FAILED_MATERIAL_NAME}' not found in library — "
+                          f"creating in-memory magenta fallback", file=sys.stderr)
+            except Exception as exc:
+                print(f"WARNING: failed to append sentinel '{FAILED_MATERIAL_NAME}': {exc}",
+                      file=sys.stderr)
+
+            if failed_mat is None:
+                # Library append failed: create in-memory magenta so export is never silently wrong
+                failed_mat = bpy.data.materials.new(name=FAILED_MATERIAL_NAME)
+                failed_mat.use_nodes = True
+                bsdf = failed_mat.node_tree.nodes.get("Principled BSDF")
+                if bsdf:
+                    bsdf.inputs["Base Color"].default_value = (1.0, 0.0, 1.0, 1.0)  # magenta
+
+            fallback_count = 0
+            for obj in mesh_objects:
+                if obj.name not in assigned_names:
+                    if obj.data.users > 1:
+                        obj.data = obj.data.copy()
+                    obj.data.materials.clear()
+                    obj.data.materials.append(failed_mat)
+                    fallback_count += 1
+            if fallback_count:
+                print(f"FailedMaterial sentinel: assigned '{FAILED_MATERIAL_NAME}' "
+                      f"to {fallback_count} unmatched objects")
 
     # Purge orphan data-blocks (palette materials mat_0/mat_1/... from the geometry
     # GLB that now have users=0 after library material substitution).